This invention relates to devices and systems for controlling the operation of a turbocharger in response to variations in engine power and altitude. More specifically, this invention relates to an improved control system for selective control of the compressor discharge pressure of a turbocharger, particularly of the type used in conjunction with engines for small aircraft.
Turbochargers in general are well known in the art and comprise a rotating assembly driven by exhaust gases expelled from an internal combustion engine. The rotating assembly includes a turbine rotatably driven by the exhaust gases and mounted on a common shaft with a compressor, whereby rotation of the turbine causes a corresponding rotation of the compressor. The compressor functions to draw in and compress air and to supply the compressed air, commonly referred to as "charge air", to the engine. Accordingly, the turbocharger operates to supply increased quantities of air to the engine to allow the engine to burn proportionally increased quantities of fuel to achieve a higher engine power output.
When the turbocharged engine is used to power an aircraft, the turbocharger and the engine are subject to frequent and substantial variations in altitude. If the turbocharger is allowed to operate in a free floating or uncontrolled manner, the power assist provided by the turbocharger is a maximum at sea level operation and full engine power. As altitude increases, the availability of air for compression decreases, as evidenced by a reduction in ambient pressure, resulting in a substantial drop-off in the pressure level of the charge air. Thus, the quantity of air supplied to the engine decreases significantly with increases in altitude to substantially decrease engine power and performance. Accordingly, it has been desirable to control the turbocharger in a manner to maintain the charge air at a substantial pressure throughout a range of anticipated altitudes of operation, and thereby prevent undesirable decreases in engine power.
Turbocharger control schemes in general typically require use of a turbocharger capable of providing charge air at a pressure substantially in excess of safe design limits for the turbocharger and/or the engine during sea level operation at full engine power. A control valve is provided to control operation of the turbocharger in a manner to limit the charge air pressure to a desired magnitude for supplying the desired power assist to the engine without exceeding the safe design pressure limit. This control valve commonly comprises a so-called waste gate valve positioned to open a passage for bypass of a portion of the engine exhaust gases around the turbine away from driving communication therewith whereby the turbocharger is rotatably driven at less than maximum speed. A controller device responsive to selected engine system parameters progressively closes the control valve upon increases in altitude to increase the proportion of the exhaust gases driving the turbine to increase turbocharger speed and maintain charge air pressure substantially at the desired magnitude.
A variety of specific controller devices are well known in the art. For example, one such controller device comprises a so-called pressure ratio controller which modulates the position of the control valve to maintain a fixed pressure ratio between ambient pressure at the intake side of the turbocharger compressor and charge air pressure at the discharge side of the compressor. Another controller device comprises a so-called pressure differential controller designed to adjust the position of the control valve to maintain a fixed pressure difference across the turbocharger compressor. However, both of these controller devices are responsive directly to ambient pressure which decreases upon increases in altitude, resulting in a corresponding and undesirable drop-off in charge air pressure upon increases in altitude. Accordingly, to prevent this reduction in charge air pressure as a function of altitude, so-called absolute pressure controllers have been designed to adjust the position of the control valve in response to altitude in a manner to maintain compressor discharge pressure substantially constant throughout a range of altitudes. For a discussion of these various types of controller devices, see S.A.E. Technical Paper 546 A, June 1962, entitled "Turbocharger Controls", authored by Robert L. Cholvin.
The turbocharger controller devices discussed hereinabove are utilized to provide a fixed upper end limit on the pressure of the charge air supplied by the turbocharger. More specifically, when the engine is operated at or near a full power setting up to a predetermined design altitude, these devices progressively close the control valve to maintain the charge air pressure at or near, but not exceeding, the upper end limit. However, at altitudes above the design altitude, the control valve is maintained in a fully closed position whereby the turbocharger operates in a free floating manner to provide charge air at a pressure which decreases nonlinearly in accordance with compressor performance capability upon further increases in altitude. Similarly, when the engine is operated at lower altitudes and at lower power settings, these controller devices move the control valve toward the fully closed position in an effort to maintain charge air pressure as close as possible to the upper end limit. For power settings wherein the control valve is closed, however, control over turbocharger operation is lost and the turbocharger operates in a free floating manner.
In an attempt to improve control over turbocharger operation, some controller devices have been developed for adjusting the position of the control valve in response to engine power setting. See, for example, U.S. Pat. No. 3,611,711. These devices operate to reduce the charge air pressure upper end limit in response to reductions in engine power setting to maintain the control valve in an at least slightly open position throughout a broader range of engine operation. However, such power-responsive devices also operate to progressively close the control valve upon increases in altitude with the turbocharger operating in a free floating manner as soon as the control valve reaches the fully closed position. Unfortunately, the altitude at which the control valve reaches the closed position is frequently less than a desired altitude of operation.
In many turbocharged engine-driven aircraft, a portion of the charge air pressure from the turbocharger compressor is utilized for pressurization of the passenger cabin in the aircraft. When the charge air pressure is substantially in excess of the desired pressure for the cabin, an outflow valve is conventionally used to prevent cabin over pressurization. However, when the turbocharger is operated in a free floating mode at sufficient altitude and/or at a relatively low engine power setting, the charge air pressure can be insufficient to maintain sea level pressurization of the cabin. In this mode, the charge air pressure fluctuates with variations in altitude, as well as other system parameters such as fuel flow, engine temperature, ambient temperature, engine speed, and the like, resulting in significant passenger discomfort.
This passenger discomfort can be minimized by preventing turbocharger operation in the free floating mode, thereby isolating the charge air pressure from fluctuations in response to parameters other than altitude. Prior art turbocharger control systems, however, have not been capable of fulfilling this need up to altitudes now required. The present invention, therefore, provides a turbocharger control system for optimizing full range control over turbocharger operation throughout ranges of altitude and engine power now required.